1. Technical Field
This invention relates generally to the field of cable television systems and, more particularly, to a method and apparatus for selectively denying service in such systems.
2. Description of the Prior Art
At a headend of a cable television system, a scrambler is normally provided to encode premium television channels. The applied scrambling precludes reception by an unauthorized converter/decoder at a connected premises. Data representing channels or tiers of programming are addressably transmitted to a particular converter/decoder and stored in an authorization memory. As a result of the addressed transmission, a subsequently transmitted program is authorized in that the decoder portion of the converter/decoder will be selectively enabled to decode the scrambled premium channel or program.
Several varieties of scrambling techniques are applied today. Each manufacturer has its own scheme which may be incompatible with others. Nevertheless, most popular scrambling systems today are based on sync suppression, in which the sync information is hidden from the television receiver's sync separator, usually by moving it to a level occupied by picture information (moving the sync tip to an equivalent picture level of 40 IRE units is common). Some systems modulate the picture carrier with a sine wave phased to suppress the horizontal blanking interval. Most systems today switch to the suppressed level at the beginning of the blanking interval and switch out at the end. Most though not all suppress the vertical blanking interval. Some systems dynamically invert the video, either on a line-by line or a field-by-field basis. This must be done carefully to avoid artifacts caused by inverting and reinverting around different levels, and by differential gain and phase of the system. Synchronization is restored either by the provision of synchronous amplitude modulated pulses on the sound carrier, digital information placed in the vertical interval or phase modulation on the picture carrier.
The provision of one scrambler per premium channel at the headend and the inclusion of a descrambler in each converter/decoder at the premises of the television receiver is particularly expensive. Furthermore, providing the converter/decoder on premises has turned out to be a great temptation to service pirates who imaginatively seek ways to receive premium channels. As a result, cable television equipment manufacturers have entered into a veritable war with such pirates resulting in complicated service authorization protocols in some instances involving multiple layers of encryption by both in-band and out-of-band data transmission further increasing the costs of the converter/decoder.
The cable industry has begun to look for new technology and to take a second look at technology developed in the early stages of development of cable television, such as the application of negative and positive traps and more recent techniques such as interdiction.
Negative trap technology is viewed by many manufacturers as a viable alternative to sync suppression scrambling methods. A negative trap is basically a narrow band reject filter. Traps are located at the drop to a subscriber's dwelling and attenuate a significant portion of a premium television channel rendering that channel unusable by the subscriber.
In the conventional embodiment, negative traps are made using L-C filter techniques. The result is a notch with finite quality Q and finite shape factor. In the case of a single channel negative trap, the center of the notch is usually located at the picture carrier frequency of the channel to be removed. This technique, sometimes called a static negative trap, requires attenuation at the picture carrier of at least 60 dB to be effective.
Negative trap systems have several advantages that make them attractive for cable television applications. One primary advantage is the ability to deliver a broadband cable television spectrum to the subscriber's converter/decoder. Conventional sync suppression systems utilize descrambling set-top converter/decoders which deliver inherently narrowband signals. Negative traps are usually mounted outside the subscriber's home (typically at the tap) and thereby minimize the exposure associated with placing hardware inside the subscriber's dwelling. Finally, some cable television operators view the negative trap as a more secure means of subscriber control than is sync suppression, as picture reconstruction is viewed as substantially more difficult.
However, the negative trap system requires hardware in locations where no revenue is generated for the cable television system. Moreover, negative traps have several severe practical limitations. L-C band reject filters have Q and shape factor limitations. Quality factors Q for L-C filters may be limited to around 30. This means that for a negative trap located at channel 8 (picture carrier at 181.25 MHz) the 3 dB bandwidth of a negative trap is typically 6 MHz (or the bandwidth of a baseband television channel). This trap would result in significant deterioration of the lower adjacent channel. Then the television receiver tuned to the lower adjacent channel, rather than having to content with a 15 dB picture-to-sound ratio, may have to contend with a sound carrier reduced an additional 6 dB or so. Frequency stability as a function of time and temperature is also a significant concern. Many cable television system operators have instituted a regular negative trap change-out program based on the assumption that after a certain period of time and temperature cycling, frequency drift will render negative traps useless.
Positive trap systems also utilize a narrow band-rejection notch filter. However, unlike negative trap systems which are used to attenuate or trap a premium channel transmission, the notch filter is used to restore the premium television channel. In this scenario, an interfering signal is placed inside the premium television channel at the cable television headend. This interfering signal is then removed at the subscriber's dwelling by use of the notch filter. Ideally this notch filter removes only the interference without removing a significant amount of television information.
Parallel to developments of different types of trapping or jamming systems, the cable industry has also evidenced a requirement to move a converter or descrambler outside of a subscriber's home to a location which is more secure from signal piracy. This concept is not new; for example, an addressable tap system was developed by Scientific Atlanta in 1983 or 1984 in which an off-premises "tap", addressed by a headband control system, gates a premium channel into the subscriber's premises. However, such products did not prove to be viable alternatives to inside-the-home signal descrambler/converters.
Another scrambling system proposed by Scientific Atlanta involved a technique of intentionally dropping a field of video signal on occasion such that an unauthorized recipient would not be able to view a properly synchronized image. Depending on how fast the subscriber's television receiver may restore synch, the image would appear to flash on and off.
A relatively recent technique for premium channel control is the interdiction system, so-called because of the introduction of an interfering signal at the subscriber's location. Most embodiments consist of a pole-mounted enclosure located outside the subscriber's premises designed to serve four or more subscribers. This enclosure contains at least one microprocessor controlled oscillator and switch control electronics to secure several television channels. Control is accomplished by injecting an interfering or jamming signal into unauthorized channels from this pole-mounted enclosure.
For efficiency's sake, it is known to utilize one oscillator to jam several premium television channels. This technique not only reduces the amount of hardware required, but also maximizes the system flexibility. The oscillator output jamming signal frequency is periodically moved from channel to channel. Consequently, the oscillator is frequency agile and hops from jamming one premium channel frequency to the next.
One such system is known from U.S. Pat. No. 4,450,481 in which a single frequency agile oscillator provides a hopping gain-controlled jamming signal output to four high frequency electronic switches. In this known system, each switch is associated with one subscriber drop. Under microprocessor control and depending on which subscribers are authorized to receive transmitted premium programming, the microprocessor selectively gates the jamming signal output of the single oscillator via the switches into the path of the incoming broadband television signal to each subscriber. Consequently, an unauthorized subscriber upon tuning to a premium channel will receive the premium channel on which a jamming signal at approximately the same frequency has been superimposed.
In the known system, it is indicated that sixteen channels may be jammed by a single voltage controlled frequency agile oscillator. With respect to one premium channel, this translates to a situation in which the jamming signal can only be present one sixteenth of the time or an approximately 6% jamming interval. The rate of hopping is also indicated at 100 bursts per second of jamming signal at a particular frequency, or a 100 hertz hopping rate. Consequently, the effectiveness of the jamming signal is questionable.
It is important that an interdiction system jamming signal frequency be placed as close as possible to the picture carrier frequency. Otherwise, adjacent channel artifacts or incomplete jamming will result. In the known system, the jamming signal is intentionally placed below the video carrier and consequently approximate to an adjacent channel producing adjacent channel artifacts.
To overcome the difficulties of such prior art interdiction systems and in accordance with parent applications U.S. Ser. Nos. 166,302 and 279,619, and concurrently filed applications referenced above and incorporated herein by reference, an improved interdiction system is described. For example, cost reduction is achieved for each subscriber unit or common circuitry associated with several subscriber unit as, for example, is provided by FIG. 2 of U.S. application Ser. No. 166,302.
In most if not all of these systems for scrambling or jamming CATV channels, situations arise when it is appropriate to deny service to a particular subscriber or even deny service generally to all subscribers. The more obvious occasions for a service disconnect are the subscriber's failure to pay for service or in response to the request of a subscriber who no longer desires service. Other occasions may relate to emergency conditions such as those caused by weather, national defense or act of God.
Typically, a disconnect command is transmitted to a subscriber decoder, converter, jammer or interdiction device over a special carrier or is imbedded in the vertical blanking interval of a transmitted video carrier. Upon receipt, the subscriber unit decodes the command, determines the command is to be performed by the particular unit and executes the command. The command is executed, for example, by turning off power to signal conversion apparatus, by operating a subscriber disconnect relay or switch or by turning off power to amplifier circuits or by other techniques known in the art.
Generally it is a principle of design of any such subscriber disconnect arrangements that as much as sixty or seventy dB of isolation be provided by the isolating device. For example, when power is turned off to signal conversion apparatus, there should only be a highly limited amplitude of signal passed to the subscriber. This is equally true for disconnect relays, powered down amplifiers, or any other known disconnect techniques. Providing such a great amount of isolation may be expensive as it may require additional equipment, circuits, or special control to accomplish.
Thus there exists a problem with present disconnect arrangements in their difficulty of achieving adequate isolation with an economical device. When service is disconnected, it is desirable to reduce the signal level reaching the subscriber's television receiver so that, under worst case conditions, the subscriber's television receiver will not receive a viewable image, no matter how sophisticated the receiver. These worst case conditions include simultaneously encountering a television receiver having excellent noise rejection performance, a low noise preamplifier installed in the subscriber's home wiring, the subscriber's being connected to a short drop cable having low attenuation, and a CATV distribution network providing a relatively high power level radio frequency input signal to the subscriber's premises. Under these conditions, it is difficult to provide an economical switch with adequate isolation which can guarantee that an unviewable image will be received.